Due to vigorous communication demands, investigations towards increasing the capacity of backbone networks are being actively conducted. With increases in transmission capacity, if wavelength-division multiplexing (WDM) is used together with raising the per-wavelength symbol rate (the modulation symbol delivery speed), the effects of wavelength dispersion and polarization mode dispersion increase sharply. Furthermore, the optical intensity for obtaining a required reception sensitivity for transmission increases, and signal quality degradation due to four-wave-mixing, cross-phase modulation, self-phase modulation, and the like produced inside the optical fiber also become problematic.
In order to solve such problems, technology that uses orthogonal frequency-division multiplexing (OFDM) on each wavelength channel and multiplexes the above with WDM is being investigated as a multiplexing technology with excellent dispersion resistance and high bandwidth utilization efficiency. With OFDM, by encoding N carriers (where N is an integer equal to or greater than 2) orthogonal to each other, the symbol rate can be lowered to 1/N compared to the case of a single carrier, and the dispersion resistance can be improved. OFDM is a general-purpose technology in the field of radio.
As a technology that OFDM modulates an optical signal, there is a method that electrically generates an OFDM signal similarly to radio and drives an optical modulator (see PTL 1). The optical system is simple if this technique is used, but since the modulator and the modulator driving unit demand bands of approximately N times the symbol rate, there is a problem in that these bands become a limiting factor.
Meanwhile, all-optical OFDM that multiplexes sub-carrier light pre-modulated by an optical modulator has been proposed (see PTL 2 and 3). As illustrated in FIG. 1, first, multiple sub-carrier light beams are generated with a multi-carrier generation circuit (optical sub-carrier generator) 101. Next, these sub-carrier light beams are discriminated into individual sub-carrier light beams with an optical separation unit 102, and after being respectively data-modulated by optical orthogonal modulators 103a and 103b, are multiplexed by an optical multiplexer 104 to obtain a modulated output. As disclosed in PTL 3, the optical separation unit 102 may comprise delayed interferometers 105, 106a, and 106b. In so doing, a high extinction ratio can be obtained, even in the case where the optical frequency grid of the WDM signals (the optical frequency interval between WDM optical signals) and the sub-carrier interval differ to some degree. Although FIG. 1 illustrates the case of two sub-carriers, the optical circuit on the transmitting side is also comparatively simple in this case, and thus is promising as a next-generation high-speed transmission technology.